Electrical servo system



A. v. BEDFORD 2,437,313

ELECTRICAL s'mvo SYSTEM Filed Dec. 30, 1944 2 Sheets-Sheet 1 INVENTOR.

mivie March 9,1948. fllflkwrlszomm -2,437,313

f .-ELECiRICAL SERVO SYSTEM 7 2 Sheets-Sheet 2 Filed Dec. '30, 1944INVENTOR.

fective to overcome the efi'ects of friction.

meansfor producing auxiliary derivatives sig-' Patented Mar. 9, 1948ELECTRICAL SERVO srs'rEM ,Alda Redford," Princeton, N. J.,.assignor toRadio or Delaware 13 Claims.

This invention relates to electrical servo systems, and moreparticularly to improvements in the art of stabilizing electrical servosystems of the type employing A.-C. control signals.

An electrical servo system, as the term is used herein, is defined ascomprising an output shaft, an electric motor coupled to said shaft,means providing a displacement signal related in some characteristic,such as its amplitude, to the difierence between the actual position ofthe output shaft and the position to which it is to be driven, and meansfor energizing the motor in response to the displacement signal, themotor tends to drive the shaft to reduce the efiect ofthe displacementsignal, and hence the motor energization, to zero. It is well known tothose skilled in the art that such systems tend to be inaccurate andsluggish if the displacement signal produces too little efiect on themotor, and

tend to overrun the correct position and fhunt if the displacementsignal is made sufilciently ef- It is common practice to combat thesedifiiculties' by adding to the displacement signal further signals whichare in efifect time derivatives of the displacement signal. If thedisplacement signal is a D.-C. voltage of variable magnitude, the properauxiliary signals are also D.-C. voltages, having magnitudes which varyas the first and higher order time derivatives of the displacementsignal. .If the displacement signal variable amplitude, the auxiliarysignals must also be A.-C. voltages, of the same frequency as thedisplacement signal, varying in amplitude in accordancewith the timederivatives of the amplitude of the displacement signal. It is importantto note that the auxiliary voltage waves are not time derivatives of thedisplacement voltage wave.

The principal object of the present invention is to provide improvedservo systems of the type wherein a variable amplitude A.-C. voltage isemployed as the displacement signal, including nals;

Another object is to provide improved methods of and means for derivingfrom a variable amplitude A.-C. voltage, further A.C. voltages varyingin their amplitudes in accordance with the time derivatives of theamplitude of said first voltage.

'A' further object is to provide an improved servo system adapted forautomatically positioning directive radio antennas in response to radiosignals picked up thereby.

in such manner that is an A.-C, voltage of Corporation oi America, acorporation r Application December 30, 1944, Serial No. 570,624

" I (Cl. 318-28) 7 2 These and other objects will become apparent tothose skilled in the art upon consideration of the followingdescription, with reference to the i accompanying drawings, of whichFigure 1 is a schematic diagram of a radio 1ocator system embodying theinstant invention,

Figure 2 is a graph illustrating a variation of the position of theoutput shaft of the system of Figure 1 with respect to the correctposition of said shaft, j i T Figure 3 is a graph illustratingvariations with time of the amplitude of a radio signal received in theoperation of the system of Figure 1,-

Figure 4 is a graph illustrating the output of the radio receiver of thesystem of Figure 1 under the conditions represented by Figures 1-3,

Figure 5 is a graph illustrating the voltage of Figure 4 after beingdelayed.

Figure 6 is a graph of the difference between the voltages of Figures 4and 5,

Figure '7 is a schematic diagram of a modification of one of thesubcombinations of the system of Figure 1,

Figure 8 is a further modification or one of the subcombinations of Fig.1,

Figure 9 is a graph illustrating the voltage of Figure 4 after beingdelayed by a different amount than that corresponding to Figure 5, and

Figure 10 is a graph illustrating the sum of the voltages of Figures 4and 9.

Referring to Figure 1, only those elements of a radio locator systemwhich are necessary to an explanation of the present invention areshown.

way switch device 3 to a pair of radiators 5 and 'I. The radiators 5 and'l are directive, and are positioned upon a supporting member 9 in suchmanner that their directive patterns overlap with the maximumdirectivity 0f the radiator 5 lying in a line to the right of theequisignal axis, and that of the radiator I lying to the left. Thesupporting member 9 is rotatable by means of a shaft I I, which iscoupled to a motor [3.

The switch 3 is coupled to a synchronous motor I 5, arranged to beenergized from an A.-C. source, not shown. The motor l5 may be arrangedto drive the switch 3 at a constant speed of 3600 R. P. M., for example,connecting the transmitter *that any known type of reversible A.-C.motor may be used. The field winding of the motor I:

is connected through a phase shifter I! to the a well known type ofphase invertor.

" reference numerals, primed.

A.C. supply. The armature of the motor I3 is connected to the outputcircuit of an amplifier IS.

'A radio receiver 2|, provided with an antenna 23, is tuned to thefrequency of operation of the transmitter I. The output circuit of thereceiver 2| is coupled to the control grid of an electron discharge tube25. The tube 25 is provided with two load resistors 21 and 29, connectedin the anode and cathode circuits respectively. A grid leak 3| is alsoprovided, with its lower end connected to a D.-C. source of biasingpotential. The tube 25, with its associated resistors, constitutes Itwill be apparent that any other known type of phase invertor may besubstituted. The phase invertor is designated generally by the referencenumeral 33 in Figure 1.

The anode of the tube 25 is coupled through a blocking capacitor 35directly to the control grid of a tube 31, and through a resistor 39.tothe control grid of a tube 4|. The cathode of the tube 25 is coupledthrough a blocking capacitor 43,

i a. delay network 45, and a resistor 41 to the con trol grid of thetube 4|. The output end of the delay network 45 is also coupled throughan amplifier 42 to the input circuit of a phase invertor 33', similar tothe phase invertor 33.

The phase. invertor 33' is coupled to the control grid of a tube 49through resistors and a delay network, in exactly the same manner as thephase invertor 33 is coupled to the tube 4|. The elements in theconnections from the phase invertor 33' which are similar to thoseassociated'with the phase invertor 33 are denoted by corresponding Thetubes 31, 4| and 49 are provided with a common load resistor 5|, whichis coupled through a capacitor 53 to the input circuit of the amplifierl9.

The time'delay networks 45 and 45' illustrated in Figure 1 are of thesame general construction as low pass filters, and in fact are-low passfilter circuits. They are designed to pass atleast the fundamentalfrequency of the output signal'of the receiver 2|, and are terminated insuch manner and provided with the proper number of sec- -tions tointroduce a delay of substantially one cycle,-i. e. if the signal is 60cycles per second, the networks 45 and 45' each cause a delay of ,4second. If desired, the networks can be designed to pass higherfrequencies as well, subject only to the condition that the required ,4second decomprising series inductors and shunt capacitors. It willbeunderstood by those skilled in the art that series resistors may be usedinstead of series inductors, and other types of networks may besubstituted for. those shown in Figure 1.

The operation of the system of Figure 1 is as follows: The transmitterprovides radio frequency output which is applied alternately to theradiators and I through the switch 3. Although the transmitter I may bemodulated, it is assumed for the sake of simplicity of description thatit merely provides a continuous wave. The operation of the servo systemis substantially the same whether or not the transmitter is modulated.Signal is radiated alternately by the. radiators 5 and If a reflectingtarget lies on a line midway between the directive axes of theradiators, the strength of the reflected signal is the same regardlessof which radiator is energized. However, if the target is to the left ofthe-equisignal line, the reflected signal is stronger while the radiatoris energized, and weaker when the radiator 5 is energized.

' lay is provided. The networks are illustrated as Figure 2 shows atypical variation of the devia# tion of the equisignal line from theline of sight to the target, such as would be caused by motion of thetarget toward the left. Referring to Figure 3, the amplitude of thereflected signal varies accordingly, the pulses ,L2,' L3 etc.representing energy transmitted from the antenna 1 becoming larger, andthe right pulses R2, R3 etc. becoming smaller. It should be understoodthat each of the pulses L and R of Figure 3 represents only theamplitude of the reflected wave. The pulse frequency is 60 cycles persecond, i. e. 60 L pulses and 60 R pulses occur each second, but thefrequency of the signal itself may be several hundred megacycles persecond. 1

The reflected signals are pickedup by the antenna 23, and amplified anddetected by the receiver 2|, providing an output voltage represented bythe graph of Figure 4. This is a 60cycle wave,

is applied to the control grid'of the tube 25, causing correspondingvaria-' tions of the anode current thereof, and hence similar variationsin the voltage drops across the resistors 21 and 29. Upon increase ofthe anode current of the tube 25, the voltageat the anode becomes lesspositive, referred to ground potential, and that at the cathode becomesmore positive; The blocking capacitors 35 and 43 pass only the A.C.components of these voltages;

thus the voltage at the control grid of the tube .31 is similar in formto that-applied to the input of the delay network 45, but out of phasewith it. The voltage input'to the delay network.

is in phase with the receiver output. The output of the delay network 45is similar to the input,

but delayed one cycle. This voltageis represented by the graph of Figure5.

The current through the resistor 39 is propor- I tional to the A.C.component of the anode voltage'of the tube 25. The current through theresistor 41 is proportional to the output voltage of the network 45.Both of these currents flow throughthe resistor 40. The voltage dropacross the resistor '40 is thus substantially proportional to the sum ofthe A.C. anode voltage of the tube 25 and the output voltage of thedelay network.

The'voltage across the resistor 40 is represented by the graph of Figure6. Since the voltage appliedthrough the resistor 39 is identical withthat represented by Figure 4, but reversed in phase, the wave of Figure6 is actually the difference between those of Figure 4 and Figure 5.Therefore, the magnitude of each pulse of the wave of Figure 6 isproportional to the difference between successive pulses of the wave ofFigure 4. The difference between successive pulses is proportional tothe time rate of change of amplitude. Thus, the amplitude of the wave ofFigure 6 is proportional to the rate of change of amplitude of the waveof Figure 4. When the wave of Figure 4 is increasing in amplitude, thederivative wave of Figure 6 is in phase with it. When the wave of Figure4 is decreasing in amplitude, that of Figure 6 is out of phasewith it.

The phaseinvertor 33' and the delay network 45' operate upon thedifferential signal appearing across the resistor 40 to provide at thecontrol grid of the tube 49 a wave corresponding in amplitude to therate of change of amplitude of the a difierential signal. This voltage,appearing across the resistor 40', is proportional to the secondderivative of the displacement. It will be apparent that furtherderivative signals may be produced by adding further networks similar tothose illustrated.

The displacement voltage is amplified by the placement component and thederivative components may be adjusted by varying the values of theresistors 39, 391,, 40 and 40'.

The composite voltage across the resistor 5| is amplified by theamplifier l9 and applied to the motor, I 3. The motor 13 is energizedthereby to' rotate the shaft I I and direct the antennas 5 and 1 towardthe target. Initially, while the displacement is increasing, thederivative component aids the displacement signal, providin increasedmotor torque to overcome friction. The second derivative component alsoaids the displacement signal while the rate of change of displacement isincreasing, to overcome inertia during acceleration of the motor l3, andbucks the displacement signal while the rate of change. ofdisplacementis decreasing. As the dis-v placement signal decreases, the firstderivative component bucks it, tending to deenergize the motor morerapidly. so that the system will coast to a stop without overshooting.The second derivative signal also bucks the displacement signal whilethe rate of change of displacement is decreasing, tending to overcomethe momentum of the moving parts. Thus, by properly proportioning theresistors to control the amplification of the derivative signals inaccordance-with the friction and mass of the motor l3 and its mechanicalload, the system may be made to operate smoothly and accurately, withoutlag or hunting:

In the operation of the system of Figure 1, the voltage appearing at theamplifier I9 comprises ,three components: an undelayed displacement Isignal, a signal similar to the displacement signal but delayed by onecycle, and-a signalsimilar to the displacement signal but delayed by twocycles. Considering the operation of the system from this viewpoint,rather than that of successive derivatives," it becomes apparent thatsome of the elements of the system of Figure 1 may be omitted withoutaltering the mode of operation of the overall system. Referringto-Flgure '7, a single phase invertor tube 33" is connected like thephase invertor 33 of Figure 1 to anode and cathode load resistors 21"and 29" respectively. 7 The cathode load resistor 29" is coupled througha blocking capacitor 33" to a delay network 45" which, like the delaynetworks of Figure 1 is designed to provide a delay of one cycle. Theoutput of the network 45" is connected to a second identical delaynetwork 45". The anode of the tube 33" is coupled through a blockingca-'. pacitor 35" and a resistor to the input circuit L through thecapacitor and the resistor 10 to.

the amplifier I9. It is transmitted in reverse phase through the networkwhich introduces a delay of one cycle, and then through the resistor Hto the amplifier Ill. The third component, delayed by two cycles,travels through both of the networks '45" and 45" and the resistor 12 tothe amplifier l9. The relative values of the resistors 10, II and 12 maybe adjusted to provide the required proportionality between the threecomponents. Thus the composite voltage applied to the amplifier I9 isidentical with that applied to the amplifier IS in the system of Figure1, although the derivative voltages are not produced separately at anypoint in the circuit. Refer to Figure 8. The circuit including the phaseinvertor 33 and delay network 45 of the systemof Figure 1 may bereplaced by a, delay v network 6 l, bridged by a resistor 63, Thenetwork 6| is similar in construction to the network 45, but designed toprovide a delay of only one-half cycle. The delayed signal is applied toa resistor 65. The original signal is also applied to the resistorthrough the resistors 63 and 61. The delayed signal is represented bythe graph of Figure 9. This is added in the resistor 65 to the originalsignal, represented by the graph of Figure 4. Owing to the half cycledelay of the network GI, each pulse of the resultant voltage isproportional in magnitude to the difference be- .tween successive leftand right pulses of the original signal. The first pulse has a magnitudeL2-R1, which, in the illustrated case, is merely L2. The secondpulse hasa magnitude R2L2, etc, Thus, the amplitude of the A.-C. component of thewave of Figure 10 is at all times proportional to the rate of change ofamplitude of the wave, of Figure 4. The low frequency component 'of thewave of Figure 10 is of no effect, since it is removed by the blockingcapacitors in the power amplifier.

Although the invention has been described in connection with anelectrical servo system associated with a radio locator, it will beunderstood that it is equally applicable to any A.-C. servo system, andmay be employed as well for any other purpose which requiresdifierentiation of the envelope of an A.-C wave. The various graphs inthe drawing illustrate rectangular waves, However, voltages ofsinusoidal or other wave forms may be used, without altering the designor operation as described.

I claim as my invention: i

1. In an. electrical servo system including an output shaft, a motorcoupled to said shaft, means for producing a A.-C. displacement signalof frequency .f cyc es per second and amplitude substantiallyproportional-to the difference between the actual angular, position ofsaid shaft and the position to which said shaft is to be driven, andmeans for applying, said displacement signal to said motor, anti-huntmeans ineluding a time delay network designed to provide a delay oflength r 2f seconds, wherein 11. is an integer, means for applying saiddisplacement signal to said delay network to produce a delayeddisplacement sig nal, ,and means for applying said delayed disaplacement signal to said motor, in addition to said originaldisplacement signal. 2. In an electrical servo system including motormeans adaptedto be energized-by an alternating current, means forproducing an A.-C. displacement signal, anti-hunt means comprising atime delay network designed to provide a delay of current to said motormeans. 1

3. In an electrical servo system including motor means adapted to beenergized by an alternating current, means for producing an .A.-C.displacement signal, anti-hunt. means comprising a time delay networkarranged to provide a delay of seconds, wherein f is the frequency ofsaid displacement signal, means for applying said displacement signal tosaid network to produce a delayed Ae-C. displacement signal, means forcombining said delayed signal with said original displacement signal inphase opposition thereto to produce a resultant alternating current, andmeans for applying said resultant current to said motor means.

4. In an electrical servo system including mo tor means adapted to beenergized by an alternating current, means for producing and utilizingin known manner an A.-C. displacement signal, anti-hunt means comprisinga time delay net work arranged to provide a delay of a seconds, whereinJ is the fundamental frequency of said displacement signal, means forapplying said displacement signal to said network to progize said motor,phase inverter means, means for applying said displacement signal tosaid phase invertor means to produce two outputs, both similar to saiddisplacement signal but 180 out of phase with each other, means forapplying one of said outputs substantially without delay to saidamplifier, and means for applying the other of said outputs to saidamplifier with a delay of seconds, wherein f is the fundamentalfrequency of said displacement signal;

6. In an electrical servo system including means for producing an A.-C.displacement signal, a motor, and an amplifier connected to energizesaid motor, means for applying said dis-- placement signal substantiallywithout delay to said amplifier, and further means for applying saiddisplacement signal to said amplifier wit a delay of seconds, wherein jis the fundamental frequency of saiddisplacement signal,

7.. In an electrical servo system including means for producing an A. C.displacement signal, a motor, an amplifier connected to energize saidmotor and means for applying said signal to said amplifier, comprising aphase inverter including two output circuits, means for applying saiddisplacement signal to said phase invertor, a voltage combining networkconnected to one of saidoutput circuits, a time delay network connectedbetween the other of said output circuits and said combining network,and means for applying the output of vsaidcombining network to'saidamplifier.

8. In an electrical servo system including means for producing an A.-C.displacement signal, a motor, an amplifier connected to energize saidmotor and means for applying said signal to said amplifier, comprisinga' phase inverter including two output circuits, means for applying saiddisplacement signal to said phase inverter, a voltage combining networkconnected to one of said output circuits, a time delay network connectedbetween the other of said output circuits and said combining network,means for applying the output of said combining network to saidamplifier, a second phase inverter including two further outputcircuits, a second voltage combining network connected to one of saidfurther output circuits, a second time delay network connected betweenthe other of said further output circuits and said second combiningnetwork, and means for applying the output of said second combiningnet-- work to said amplifier,

9. The invention as set forth in claim 8 wherein said delay network isdesigned to provide a delay of seconds, wherein j is the fundamentalfrequency of said displacement signal.

10. The invention as set forth in claim 7 wherein said delay networksare each designed to provide a delay of seconds, wherein f is thefundamental'frequency of said displacement signal.

11. The invention as set forth in claim 13 wherein said delay network isdesigned to provide a delay of seconds, wherein j is the fundamentalfrequency of said displacement signal.

12. The invention as set forth in claim 13 wherein said delay networksare each designed to provide a delay of seconds, wherein f is thefundamental frequency of said displacement signal, and n is an integer.

13. In an electrical servo system including means for producing an A.-C.displacement signal, a motor, an amplifier connected to energize i saidmotor and means for applying said signal to said amplifier, comprioing avoltage combining circuit in the input circuit of said amplifier. meansfor applying said signal directly to said combin.

deriving a. difierence voltage whose amplitude is proportional to thedifl'erence between the ampiitude of each cycie oi. saiddisplacementsignel and the next succeeding cycle, means for applying 10nnnnmcns man The following references are of record in the ing circuit,means including a. delay network for 5 me P n UNITED STATES PATENTSNumber said diflerence voltage to said combining circuit, m and meansfor appl the output of said combining circuit touid amplifier.

. AIDA V. BEDFORD.

Name

Date Scott Oct. 6, 194-2 Hull Mar. 4, 1941 Aiexandereon Sept. 22, 1925Retail. Jnn,9,140

